Control of High Voltage Direct Current (HVDC) power transmission systems, denoted DC grids in the following, is a challenging task and HVDC transmissions have up to date essentially equaled point-to-point two station transmissions. However, the introduction of voltage source converter (VSC) technology has provided the possibility of controlling larger DC grids, i.e. controlling multi-terminal DC grids.
FIG. 1 illustrates a simplified single line diagram of a VSC converter. The VSC converter is on its AC side connected to a point of common coupling (PCC) in an ac network and on its dc side connected to a dc transmission.
Conventionally, the VSC converter station is controlled by controlling AC voltage UPCC at the PCC or the amount of reactive power injected into the AC network at the PCC, and controlling DC voltage (Id at the DC terminal of the converter or the amount of active power injected into the AC network (PPCC).
For a typical point-to-point HVDC transmission based on VSC technology, one station is controlling the local voltage Ud, striving to keep a constant dc-voltage, and the other station is controlling the local PPCC, adjusting its DC voltage so as to reach a set reference power PPCCref. The DC voltage controlling station outputs any amount of power within design limits such that the power controlling station reaches its control objective, i.e. the power of the power control station tracks its power reference.